A human-in-the-loop simulation was conducted that examined separation assurance across four progressive future time frames. Decision support, traffic density, separation assurance roles and responsibilities, and aircraft equipage mix were varied across conditions. In a near-term condition, these factors were set to approximate current day operations. In contrast, the most far-term condition involved two times current traffic, full air-ground data communications equipage, and automated conflict resolution working independently. The variation across the four conditions provided an opportunity to explore the pattern of reported controller workload, and what factors contributed to any observed differences. Despite increasing levels of traffic, results showed that mean workload ratings did not differ across conditions with the exception of the furthest term condition which was significantly lower. However, additional analyses were conducted that examined the relationship between workload and the varying traffic characteristics per condition. Although each condition had different significant contributors to workload, the one consistent contributor to workload across each condition was the number of conflicts. This result highlights the importance of work being done to develop the concepts and automation necessary to progressively balance the allocation of separation assurance functions between automation and the air traffic controller of the future.